1. Field of the Invention
The present invention relates to an automatic gain control (AGC) type demodulation apparatus in a digital radio communication system.
2. Description of the Related Art
A prior art AGC type demodulation apparatus is generally constructed by an orthogonal demodulation circuit for receiving an analog input signal to generate a first I-signal (I-axis component or I-channel) and a first Q-signal (Q-axis component or Q-channel) orthogonal to each other. An analog/digital (A/D) converter circuit performs an A/D conversion operation upon the first I-signal and the first Q-signal to generate a second I-signal and a second Q-signal. A first AGC circuit suppresses the difference in amplitude between the second I-signal and the second Q-signal to generate a third I-signal and a third Q-signal. A complex multiplier rotates the third I-signal and the third Q-signal by a phase offset angle to remove the frequency and phase offset components of a carrier wave included in the third I-signal and Q-signal to generate a fourth I-signal and a fourth Q-signal. In this case, the phase offset angle is obtained by a loop circuit formed by a phase detector, a low-pass filter and a numerical control oscillator. A second AGC circuit compensates for the difference in amplitude between the fourth I-signal and the fourth Q-signal and one normal signal point in accordance with one amplitude error of the fourth I-signal and the fourth Q-signal with respect to one normal signal point. This will be explained later in detail.
In the prior art AGC-type demodulation apparatus however, an accurate automatic gain control cannot be expected. As a result, the I-signal and Q-signal regenerated by the second AGC circuit still show a circular locus around the normal signal point. In addition, since the combination of the two AGC circuits is large in size, the AGC demodulation apparatus is increased in size.
It is an object of the present invention to provide an AGC-type demodulation apparatus capable of an accurate automatic gain control.
Another object is to decrease an AGC-type demodulation apparatus in size.
According to the present invention, in an AGC-type demodulation apparatus, an orthogonal demodulation circuit receives an analog input signal to generate a first I-signal and a first signal orthogonal to each other. An A/D converter circuit performing an A/D conversion operation upon the first I-signal and the first Q-signal to generate a second I-signal and a second Q-signal. An automatic gain control (AGC) circuit suppresses amplitude errors of the second I-signal and the second Q-signal to generate a third I-signal and a third Q-signal. A complex multiplier removes frequency and phase offset components of a carrier wave included in the third I-signal and the third Q-signal to generate a fourth I-signal and a fourth Q-signal. A phase detector detects first and second amplitude errors of the fourth I-signal and the fourth Q-signal, respectively, with respect to one normal signal point and calculates a phase error of the fourth I-signal and the fourth Q-signal. A numerical control oscillator converts the phase error into first and second angle signals orthogonal to each other. The AGC circuit controls the amplitude errors of the second I-signal and the second Q-signal in accordance with the first and second amplitude errors and the first and second angle signals. Thus, a single AGC circuit is provided in the AGC-type demodulation apparatus.